To alienation of the master's property his consent was always necessary. With that consent, which might be by ratification, or by a general authorisation if wide enough in its terms, the slave could alienate anything. He could not of course make a cessio in iure, because this was in form litigation, but apart from that the form is immaterial. There is indeed little authority for mancipatio by a slave, but what little there is is in favour. Julian contemplates the transfer of proprietas in a slave, by a slave with authority, but it is possible that the text, which speaks of traditio, may have been originally so written, so that the reference would be only to Praetorian ownership. Of course the dominus could not authorise the slave to do what would have been unlawful had he done it himself. Thus a slave could not validly make a donatio to his owner's wife. Without authority, the slave was powerless: he could not transfer dominium. If he sold and delivered, possession passed but no more, and the taker, if he knew that there was no authority, could not prescribe, and was indeed a fur. Money lent, citra voluntatem, could be vindicated, as could money paid by a fugitive slave for the concealment of himself or his theft.

Similar rules applied where, having authority, he exceeded it. Where A owed B 10 ex fideicommisso and 10 on an independent obligatio naturalis, and a generally authorised slave paid 10 expressly towards the whole debt, 5 could be vindicated, as a general authority to pay is not held to apply to natural obligations.